ABSTRACT:W. D. Hamilton was one of
the first to suggest that coevolution with parasites
could lead to selection for sexual as opposed to asexual
reproduction. I briefly trace the history of Hamilton's
thinking on this subject, particularly his concern about
the strength of parasite-mediated selection required to
compensate for the two-fold cost of sex. I also discuss
recent studies from a freshwater snail that were
designed to test the expectations and necessary
conditions for the parasite hypothesis of sex (also
known as the Red Queen hypothesis). The snails are
useful for these studies, because both sexual and
asexual modes of reproduction are known to occur. The
results of these studies are surprising consistent with
the Red Queen hypothesis in showing that (1) sexual
populations are more common where virulent parasites
also exist, and that (2) parasites are adapted to
infecting local populations of their hosts as well as
common genotypes within these populations. Finally, I
discuss how host-parasite coevolution might aid the
accumulation of deleterious mutations in clonal host
populations, and how this process might prevent the
elimination of sexual females by a genetically diverse
assemblage of clones.